GPIO - this is a pin that is used by the sensor to indicate that data is ready. It's useful for when doing continuous sensing. Note there is no level shifting on this pin, you may not be able to read the 2.8V-logic-level voltage on a 5V microcontroller (we could on an arduino UNO but no promises). Our library doesn't make use of this pin but for advanced users, it's there!

All the latency (delay) from the project deleted !
The Adafruit library works fine - but data gets slow, and I these values must be averaged, the readings jump.
But the SparkFun library is very fast and allows you to connect through the multiplexer, several sensors to one controller.
With the Adafruit library, you can connect only one sensor.